Well, according to "Gunbroker.com" a used Sheridan Blue Streak can be had for under 100 bucks, in "well used" condition. What condition is the one you were offered? The Blue Streak is fairly well legendary as a good gun to own, and if it's in good shape, the value may increase. I don't know about you, but I really don't care about such things. Shoot the Blue Streak, and see how you like it. Scoping one (while not impossible) could be a hassle.
https://www.bakerairguns.com/product/one-piece-scope-mount-benjamin-397-392-air-rifles/ Check this thing out. Mounts to the receiver without risk of stressing things (these guns are all brass construction). I'm not certain it would fit the Sheridan you're considering, you'd have to check with the maker. That said, your Gamo is capable of significantly more power, I'd bet. And with practice, I'm certain is capable of shooting accurately. Springers are a bit of a pain, look up "artillery hold" on this site and others. The Sheridan may be able to be upgraded for more power (Mac1, etc), but will likely never shoot at the kinds of speeds your Silent Cat hits. That said, your Cat will likely be limited for future modifications, where the Sheridan is not.
Something nice about pumpers is the fact you can vary the muzzle velocity from shot to shot. 2-3 pumps for indoor shooting safety, or 4-8 pumps for outdoor power. The Gamo won't do that. Lastly, .20 cal ammo is expensive, at least compared to the more common calibers (.177 & .22). There are a lot more choices now, as the caliber has gained popularity in the last few years. In the end, as I said at the beginning of this long post (sorry), you've got to shoot the Sheridan and see if you like it. Also, determine which will function better in the uses you intend (hunting, target shooting, plinking, pest control, etc). Hope this helps, and best of luck!